As an emission type electronic displaying device, there is an electroluminescence device (ELD). As materials constituting the ELD, there is an inorganic electroluminescence element or an organic electroluminescence element. The inorganic electroluminescence element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element. An organic electroluminescence element has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by deactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several to several decade volts. Further, the element has a wide viewing angle and a high visuality since the element is of self light emission type, and the element is a complete solid element, and the element is noted from the viewpoint of space saving and portability.
However, in the organic EL element for practical use, an organic EL element is required which efficiently emits light with high luminance at a lower power.
In U.S. Pat. No. 3,093,796, there is disclosed an element with long lifetime emitting light with high luminance in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives are doped with a slight amount of a fluorescent compound.
An element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (Japanese Patent O.P.I. Publication No. 63-264692), and an element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (Japanese Patent O.P.I. Publication No. 3-255190).
When light emitted through excited singlet state is used, the upper limit of the external quantum efficiency (next) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%. Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Prinston University (M. A. Baldo et al., Nature, 403, 17, p. 151–154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made. As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits the same performance as a cold cathode tube, and can be applied to illumination.
In order to improve luminance and emission lifetime of the organic EL element, proposal has been made in which a hole blocking layer, inhibiting migration of holes from the light emission layer, is provided between the light emission layer and the cathode. This hole blocking layer can efficiently accumulate holes in the light emission layer and improve a recombination probability of electrons and holes, resulting in light emission with high efficiency. It is reported, for example, in Japanese Patent O.P.I. Publication Nos. 8-109373 and 10-233284, that a phenanthroline derivative and a triazole derivative are effectively used alone as a hole blocking material of the hole blocking layer. In Japanese Patent O.P.I. Publication No. 2001-28405 is disclosed an organic El element with long lifetime in which a specific aluminum complex is used in the hole blocking layer. It has been reported (for example, in Twelfth OyobutsuriGakkai Gakujutsukoen Kai Yokoshu 12-a-M7 or in Pioneer Gijutsu Johoshi, Vol. 11, No. 1) that a red or green light emission organic EL element employing a phosphorescent compound, when a hole blocking layer is incorporated in it, exhibits an inner quantum efficiency of approximately 100% and a lifetime of twenty thousand hours. However, there is room to be improved as for emission luminance.
There is an example in which a phosphorescent compound emitting a blue to blue-green color light is used as a dopant compound and a carbazole derivative such as CBP is used as a host compound, but the external qauntum efficiency of this example is around 6%, which provides unsatisfactory results, although the phosphorescent compound is used (for example, Twelfth OyobutsuriGakkai Gakujutsukoen Kai Yokoshu 12-a-M8, or Adachi et. al., “App. Phys. Lett., Vol. 79, p. 2082).